WASHINGTON — Call it the rule of unintended consequences — drop your guard because one threat goes away and an unexpected menace jumps up and smacks you. And new research shows it even applies to African acacia trees.

For thousands of years these thorny shrubs have provided food and shelter to aggressive biting ants, which protect the trees by attacking animals that try and eat the acacia leaves.

Called mutualism, it's a good deal for both the trees and the ants.

Scientists studying the decline in large animals in Africa wondered what would happen if they no longer were eating the leaves. So they fenced off some of the acacias, so elephants, giraffes and other animals couldn't get to them.

Surprisingly, after a few years the fenced-in trees began looking sickly and grew slower than their unfenced relatives.

It turns out that without animals eating their leaves the trees no longer bothered to take care of their ants — they reduced nectar production and made fewer swollen thorns that the ants could live in.

The result: The protective ants either began damaging the plant or were replaced by other insects that ate holes in the bark.

"Although this mutualism between ants and plants has likely evolved over very long time-scales, it falls apart very, very rapidly," said Todd Palmer, an assistant professor of zoology at the University of Florida.

"Over the course of only 10 years, we found that when mammals could not eat plants, the plants began to have less use for the ants, and therefore began to reduce their 'payments' to the ants, in the form of nectar," Palmer, who is currently in Kenya, explained in an interview via e-mail. Palmer's findings are reported in Friday's edition of the journal Science.

"If you had asked me 10 years ago 'what would happen if you took large mammals out of the system,' I would have answered 'I'll bet the trees would be really happy!'" he said.

But instead, because the browsing animals are the driving force behind the tree paying out benefits to the ants, when the payments diminish, the ants that protect the tree begin to starve and its colonies become smaller.

Some ants reduced their defensive behavior and began tending colonies of scale insects that bore into the plants and extract sugars. Others were replaced by other ant species that eat elsewhere and encourage the presence of wood-borer beetles, which eat holes in the trees that the ants can then use as home.

"So, that's one lesson from the research, to me: The human-induced decline of big herbivores in Africa can have some pretty dramatic and non-intuitive consequences for the communities in which these large mammals live," Palmer said.

Ted R. Schultz, a research entomologist at the Smithsonian Institution's National Museum of Natural History, said that removal of the browsing animals turns out to be worse for the plant "is surprising and it's not the kind of thing anybody would have been likely to predict in advance."

Schultz, who was not part of Palmer's research team, said the report shows that mutualisms are finely balanced and complex.

"The system reported here is a balance of a number of players — the trees, the browsing mammals, the main ant and three other ant species, with the ants all competing for the trees. Remove one of the players — the browsing mammals — and all the other moving parts rearrange themselves in a way we hardly could have predicted." he said.

So, can the trees recover their protective ants if large animals start nibbling on then again?

Palmer means to find out by exposing the trees again to browsing, "to see how quickly trees will re-induce their investments in symbiotic ants, and in turn, whether such reinvestment will be enough and in time, or too little, too late."

The research was funded by the U.S. National Science Foundation, the Smithsonian Institution, National Geographic Society and the African Elephant Program of the U.S. Fish and Wildlife Service.

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